1. Field of Invention
The present invention relates to a backlight module and a liquid crystal display device, and more particularly to a backlight module having a backplate capable of extending according to the expansion of a light guide plate and a liquid crystal display device using the same.
2. Related Art
In the field of the backlight module of the liquid crystal display device, with an increase in the size of screen of the liquid crystal display device, miniaturization is also required; therefore, the edge-illumination type backlight unit is a first priority choice. In order to meet the specification of the illumination of screens, the numbers of the light source module and the driving power are increased accordingly, such that thermal energy produced by the light source module increases as the total power thereof increases. The light source of the edge-illumination type backlight unit is disposed on the side wall of the backplate, such that the thermal energy is concentrated and the members of the backlight module expand by heat to pressurize each other, and then the members are damaged.
Referring to FIGS. 1 and 2, FIG. 1 illustrates a conventional backlight module, and FIG. 2 is a cross-sectional view of the conventional backlight module along the line A-A of FIG. 1.
A backlight module 1 comprises a backplate unit 11, two light source units 12, two positioning and heat dissipating units 13, a reflection unit 14, a light guide unit 15 and an optical film unit 16. The two light source units 12 are disposed on a side wall of two positioning and heat dissipating unit 13, respectively. Each positioning and heat dissipating unit 13 is equipped on a lateral side of the backplate unit 11, the reflection unit 14 is disposed on the two positioning and heat dissipating units 13, and the light guide unit 15 is disposed between the reflection unit 14 and the optical film unit 16, wherein each light source unit 12 has a light emitting surface 121, the light guide unit 15 comprises two light entrance surfaces 151, the number of the light entrance surface 151 is the same as the number of the light source unit 12, a light coupling distance (d) exists between each light emitting surface 121 and a corresponding light entrance surface 151. A height of the light entrance surface 151 is adjusted to the same as a height of the light emitting surface 121 of the light source unit 12 by the two positioning and heat dissipating units 13.
FIG. 3 is a cross-sectional view of the conventional backlight module along the line A-A of FIG. 1 after the conventional backlight module deforms by heat. As the foregoing, after the thermal energy produced by the two light source units 12 conducts to the reflection unit 14, light guide unit 15, and the optical film unit 16, each unit expands by heat. Because each unit has different heat expansion ratios and the heat expansion ratio of the light guide unit 15 is larger than the heat expansion ratio of the backplate unit 11, the width of the light guide unit 15 is larger than the width of the backplate unit 11 after heat expansion. When the light guide unit 15 expands, the light source unit 12 is pressurized to be broken by the light guide unit 15. Meanwhile, the light guide unit 15 warps and deforms, such that the uniformity of the illuminance of the backlight module 1 is decreased.
The width of the light guide unit 15 is larger than the width of the backplate unit 11 after heat expansion, such that the light guide unit 15 expands by heat to pressurize the light source unit 12; therefore, how to avoid the light source unit to be damaged due to the expansion of the light guide unit is an important problem to be solved.